Library of Congress Cataloging-in-Publication Data
Pediatric neuroendocrinology / volume editors, Sandro Loche ... [et al.]. p. ; cm. -- (Endocrine development, ISSN 1421-7082 ; v. 17)
Workshop, May 17-19, 2009, Villasimius (Cagliari), Italy. Includes bibliographical references and indexes. ISBN 978-3-8055-9032-1 (hardcover : alk. paper)
1. Pediatric neuroendocrinology--Congresses. I. Loche, Sandro. II. Series: Endocrine development, v. 17. 1421-7082 ; [DNLM: 1. Puberty--physiology--Congresses. 2. Growth Hormone--physiology--Congresses. 3. Pituitary-Adrenal System--physiology--Congresses. W1 EN3635 v. 17 2010 / WS 450 P371 2010]
RJ418.P436 2010 618.92 8--dc22
Bibliographic Indices. This publication is listed in bibliographic services, including Current Contents®
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175 Resistin: Regulation of Food Intake, Glucose Homeostasis and Lipid Metabolism
Nogueiras, R.; Novelle, M.G.; Vazquez, M.J.; Lopez, M.; Dieguez, C. (Santiago de Compostela)
185 Hypothalamic Obesity
Hochberg, I.; Hochberg, Z. (Haifa)
197 Neuroendocrine Consequences of Anorexia Nervosa in Adolescents
Misra, M.; Klibanski, A. (Boston, Mass.)
215 Author Index
216 Subject Index
Preface
Pediatric neuroendocrinology is an important field of clinical and scientific interest, which has rarely been addressed as a single entity. Consequently, this is a particularly welcomed volume. In this issue of Endocrine Development, an eclectic group of highquality clinicians and scientists has been assembled to provide focussed updates of their particular fields of interest. The scope of pediatric neuroendocrinology and its potential disturbances is wide and has direct relevance to both pediatric and adult endocrinology, as major pediatric pathology is likely to have implications in adult life.
The principle hypothalamic-pituitary axes with discussion of the neurobiology and its disturbances in a range of topics including neurogenesis, sleep and its abnormalities, sexual differentiation, onset of puberty, and stress are all covered here. The physiology and pathophysiology of ghrelin, leptin and kisspeptin are described as well as the pharmacological effects of modulating the hypothalamo-pituitary-adrenal axis. Contributions with a more clinical orientation include those on disease entities such as abnormal puberty, central adrenal insufficiency, pituitary tumors and Cushing’s disease. Advances in investigations such as neuroimaging and the molecular characteristics of pituitary adenomas are provided by the leaders of their respective fields. Finally, two chapters on the extremes of disordered energy balance, namely hypothalamic obesity and anorexia nervosa, highlight the endocrine disturbances in and the therapeutic options for these serious conditions.
This volume covers a wide range of topics in pediatric neuroendocrinology and informs the reader of the latest scientific developments as well as the diagnostic and molecular techniques and therapeutic options available today. We believe that the volume will benefit scientists and clinicians involved in the care of children with neuroendocrine disorders.
S. Loche, M. Cappa, L. Ghizzoni, M. Maghnie, M.O. Savage
Loche S, Cappa M, Ghizzoni L, Maghnie M, Savage MO (eds): Pediatric Neuroendocrinology. Endocr Dev. Basel, Karger, 2010, vol 17, pp 1–10
The Transcriptome and the HypothalamoNeurohypophyseal System
Charles Colin Thomas Hindmarch David Murphy
Henry Wellcome Laboratories for
Integrative Neuroscience and Endocrinology,
University of Bristol, Bristol, UK
Abstract
The hypothalamo-neurohypophyseal system (HNS) is a highly specialised region of the brain that is comprised of the magnocellular neurons of the paraventricular (PVN) and supraoptic (SON) nuclei, the axons of which project to the neural lobe of the pituitary. The PVN and the SON are involved in a broad spectrum of activities including, but not restricted to, osmotic regulation, cardiovascular control, parturition and lactation, energy homeostasis and the stress response resulting in a functionrelated plasticity of these tissues, allowing them the modulation necessary to reply to the physiological demands in an appropriate manner. We hypothesise that the HNS response to physiological stimulation is underpinned by changes in gene transcription. Affymetrix microarrays with 31,099 probes representing the total rat genome, were interrogated with RNA targets from SON, PVN and the neuro-intermediate lobe dissected from naïve rats as well as those responding to physiological and pathological cues. The data generated are comprehensive catalogues of genes that are expressed in each tissue, as well as lists of genes that are differentially regulated following changes in the physiological state of the animal.
The brain is often described as the most complex ‘thing’ in the universe. That this most complicated ‘thing’ is the result of a process typified by its simplicity whereby graduated changes in operation provide a substrate for selection pressure is remarkable enough. Even more remarkable is that the complexity of the brain and the animal as a whole is dependant on the information encoded by just 25,000 genes, a number that does not differ significantly between the rat and the human. The plasticity of the genome to respond to changes in the internal and external environment is mediated by the expression of each individual gene, in each cell, in concert. Collectively these transcript expressions may be defined as the transcriptome; the total transcript expression of the genome.
Work in our laboratories has sought to comprehensively catalogue the transcriptome of hypothalamic structures that are involved in osmoregulation. To this end, microarray gene chips have been employed to measure the simultaneous expression
levels of the whole genome. Gene chip data rely on the ratio between the hybridisation of labelled target mRNA from a normal and a treated biological unit to multiple probes representing a specific transcript. This ratio is related to the level of mRNA expression between the two units with the signal from each unit being representative of the relative amount of a particular transcript in each condition (fig. 1). The desired end point of the microarray study is not necessarily to determine which genes are being expressed in a particular paradigm (though this is an obvious benefit), but to gain a perspective on the relative expression of each gene simultaneously in response to the paradigm. The hypothesis assumed by microarray experiments is that the biological environment is under transcriptomic control and that co-expression of different gene populations cooperatively maintain the stability of the biological environment.
Osmoregulation
Osmotic stability is aggressively defended in mammalian organisms [1] that must maintain a wet internal environment in a dry external environment; fluids lost through excretion, perspiration or expiration must be replaced quickly. Osmoregulation is a highly conserved mechanism that provides a means by which an organism can maintain a constant prescribed level of water and salts within the intra- and extracellular fluid. The path of least resistance here is a behavioural adaptation to replacing water by actively seeking out water. However, since water is not always readily available, this mechanism is complemented by a physiological approach to limit the amount of water lost in times of osmotic stress. The main mechanism involves the brain peptide hormone vasopressin (Avp) which acts on the kidney to conserve water.
Detection of Hyperosmolality
Dehydration effectively increases the relative concentration of sodium and other electrolytes in the extracellular fluid with the result that water is drawn from cells. Although all cells are subject to this cellular dehydration, certain cells are particularly responsive to it and are able to communicate the change in osmolality to the brain. The subfornical organ (SFO) is a circumventricular organ that is capable of detecting changes in osmotic status, is unprotected by the blood-brain barrier (BBB) and is well connected with the hypothalamus. In contrast with regions protected by the BBB, the SFO is well vascularised with highly fenestrated capillaries allowing effective diffusion of peripheral signals that the SFO can ‘taste’. Direct connections between the SFO and the vasopressinergic magnocellular neurons of the supraoptic (SON) and paraventricular (PVN) nuclei have been demonstrated [2]. Moreover, these connections are functionally active and responsive to changes in osmolality since intravenous injection of NaCl in the rat regulates the immediate early gene c-fos mRNA and Fos protein in the SFO, the PVN and the SON.
· Murphy
RNA extraction
RNA amplification and labeling
Validation Validation
Hybridisation of labeled RNA to microarray
Analysis of microarray
(1) Normalisation
(2) Identification of probes flagged as present in all 5 control chips
(3) Identification of probes flagged as present in all 5 dehydrated chips
(4) Merging of control and dehydrated present lists
(5) Filtering data for those genes whose expression changes by at least 2-fold
(6) Perform statistical test on >2-fold gene lists to identify those that significantly change as a result of dehydration
Fig. 1. a Typical microarray workflow, where mRNA extracted from tissue dissected from either a control or a treated animal is amplified, labelled and hybridised to a gene chip. Each chip, control or treated, results in signal intensities for each of the genes that are represented on the array. By comparing the signals of control and treated datasets, a signal ratio is generated that can be used for fold-change filtering and statistical testing. Identification of gene targets using bioinformatical approaches are validated using molecular and physiological approaches. b A generalised microarray experiment analysis [15]. c The Affymetrix GeneChip® Rat Genome 230 2.0 microarray is comprised to 31,099 probesets, representing 30,000 transcripts from over 28,000 rat genes. d Each gene is represented by 1 or more probesets. The signal intensity of the probeset is the function of multiple probes that are a perfect match (PM) to the RNA sequence. Each PM sequence is complemented by an mismatch (MM) probe, an identical sequence with a single nucleotide-difference. This MM can be used for signal correction. e Immunohistochemistry Dab staining using an anti-vasopressin (Avp) antibody. Avp immunoreactivity can be seen in the (i) lateral magnocellular portion of the paraventricular nucleus (PVN) seated at the top of the 3rd ventricle (3V), and (ii) the supraoptic nucleus (SON) which is at the boundary of the optic tract (opt) and the suprachiasmatic nucleus (SCN) where vasopressingergic neurons are also present.
IIIIIIIVVVI
This Fos expression can be abolished in the PVN and SON but not the SFO when the connection between the SFO and the hypothalamus is severed [3]. Also, destruction of the SFO results in the partial abolishment of osmotically induced Avp release [4]. For total abolishment, however, destruction of the entire lamina terminalis is required [5].
The Hypothalamo-Neurohypopyseal System
The hypothalamo-neurohypophyseal system (HNS) is a highly specialised region of the brain that is comprised of the magnocellular neurons of the PVN and the SON that project their nuclei to the posterior lobe of the pituitary (PP). The PVN and the SON are involved in a broad spectrum of activities including, but not restricted to, osmotic regulation, cardiovascular control, parturition and lactation, energy homeostasis and the stress response [6–10] resulting in a function-related plasticity of these tissues and allowing them the modulation necessary to reply to the physiological demands in an appropriate manner. Seated in a position immediately lateral to the boundary of the optic tract (fig. 1), the SON is a relatively homogeneous population of large (10–40 μm cell body diameter) and densely packed magnocellular neurons, the axons from which proceed to the neural lobe of the pituitary where they terminate [11]. The main known role of the magnocellular neurons is confined to the appropriate synthesis and secretion of two closely related hormones; vasopressin (Avp) and oxytocin (Oxt) that are involved in osmoregulation and reproductive duties, respectively. While the SON is a relatively homogeneous population of MCNs that terminate in a single hypophyseal location, the PVN is a rather more complicated structure. Situated slightly caudal to the SON, the PVN is located on either side of the third ventricle and may be split into eight discrete subdivisions of either large magnocellular or smaller (10–15 μm cell body diameter) parvocellular neurons (fig. 1). The parvocellular neurons project in a more diverse manner than that of the magnocellular neurons, terminating in numerous central sites and therefore being involved in a wide range of biological functions. For example in response to stress, the parvocellular regulation of corticotropin-releasing factor (CRF) and vasopressin together with their subsequent release into the anterior pituitary portal blood system results in adrenal release of glucocorticoids via adrenocorticotropin hormone stimulation [7]. Also, through parvocellular projections to sympathetic preganglionic motor neurons of the rostrol ventrolateral medulla (RVLM) and the intermediolateral cell column, PVN is able to directly influence sympathetic nerve traffic [9].
Function-Related Plasticity
Upon activation, the neuronal populations of the SON and PVN undergo a dramatic event called function related plasticity, defined by Hatton as the power that
fluctuating physiological conditions have to reversibly alter the structural relationships among the various cell types as well as the functional pathways over which information is transmitted [12]. In line with this definition various stimuli including: dehydration, decreases in blood pressure, late stages of pregnancy, parturition and lactation, induce morphological, electrical and biosynthetic changes in the SON and PVN that are fully reversible upon removal of the stimulus [12]. These morphological changes are accompanied by biochemical events such as a strong activation of the cyclic adenosine monophosphate (cAMP) pathway in both SON and PVN [13] and transcriptional events that extend beyond simple Avp and Oxt genesis [14].
A Comprehensive Description of the HNS Transcriptome
That the SON and PVN undergo morphological and biosynthetic changes as a result of appropriate stimulation implies that function-related plasticity is necessary to create a favourable environment for the proportional and appropriate delivery of the hormone payload. We hypothesise that the elegant plasticity of the hypothalamus in response to dehydration is under the direct control of gene transcription and have therefore catalogued gene expression within the male rat SON, PVN and neurointermediate lobe of the pituitary (NIL). For each tissue we generated a list of genes that are statistically considered to be present in each of the 5 independent experimental chips in the control or dehydrated state [15]. These lists were then combined so that a single list of genes considered present in either the control or dehydrated animal could be used as a basis for further filtering and statistical analysis. In total, 183 genes were significantly (p < 0.05) regulated by greater than 2-fold in the SON. Of these 183 transcripts, the literature confirmed that 13% of them have already been described in the SON and 6% of the 183 are specifically regulated as a result of osmotic cues. It is also interesting to note that 17 of the transcripts identified as being significantly regulated in the SON as a consequence of the hyperosmolality in this study appear to be regulated in an opposite direction by hypo-osmolality induced by pharmacological manipulation with Avp [16]. When the PVN data were subjected to the same fold change threshold and statistical testing as the SON, it appears that only 12 genes are regulated. Given the common function of the SON and PVN it is perhaps surprising that such a great disparity in the number of regulated genes exists, until one remembers that in contrast to the SON, the PVN is a heterogeneous population of neurons involved in multiple biological functions that extend beyond osmoregulation. It is likely that a combination of noise generated from parvocellular neurons involved in, for example, the stress response, together with the stringent statistical cut-offs we have applied to our data has resulted in the lower number of regulated genes noticed in this tissue; a 1.5fold cut-off results in a greater number of significantly regulated genes. To further
investigate the phenotypic differences that exist between the SON and PVN, we compared the transcriptome data from the two tissues to identify which population of mRNAs in the PVN might be specifically parvo- or magnocellular in nature. The data were arranged so that those genes that have an expression level of greater than 5-fold in either tissue under either euhydrated or dehydrated states were revealed. In the control state, several genes known to be confined to parvocellular regions were revealed including corticotropin-releasing hormone [17].
When the list of genes regulated by greater than 2-fold following dehydration were compared between the SON and PVN, 7 of the 12 PVN genes are commonly regulated in the SON, presumably confirming their magnocellular credentials. One of these genes, gonadotropin-inducible ovarian transcription factor 1 (Giot-1), provided an ideal candidate for validation. Transcription factors are mature proteins capable of binding to the promoter sequence of a specific gene, thus regulating its transcription. The importance of transcription factors to our hypothesis that transcriptional events underpin hypothalamic plasticity is immediately clear. With this in mind, the SON data were statistically analysed without a fold-change cut-off being applied. This resulted in 2,453 transcripts of which 38 were identified as mRNAs that encode known transcription factors [18]. Using the subjective criteria of novelty and abundance, 5 transcription factors; Giot-1, Giot-2β, cAMP-responsive element-binding protein 3-like 1 (Creb3l1), CCAAT/enhancer-binding protein-β (Cebpb) and activating transcription factor 4 (Atf4) were selected for validation. Using in situ hybridisation histochemistry, the regulation of all 5 transcription factors was confirmed in the SON following dehydration. In the PVN, Giot-1 Creb3l1 and Atf4 were all significantly regulated following dehydration whereas Giot2 and Cebpb only just failed to reach significance. Presumably these transcription factors are expressed following chronic (72-hour) dehydration so that either a proportional response to the continued osmotic stress is maintained or a recovery may be initiated upon rehydration. Additionally, we confirmed that the upstream signalling pathways that regulate the Giot-1 transcript in the HNS are cAMP dependent. The activity of the Giot-1 proximal promotor has already been demonstrated to be induced by cAMP intracellular pathways through a cAMP-responsive element (CRE) site [19]. We have demonstrated that unilateral injection of an adenoviral construct encoding PKIα, a specific inhibitor of protein kinase A, significantly reduced the upregulation of Giot-1 noticed in the dehydrated PVN [18]. Interestingly, the Giot-1 promotor has also been shown to be the target of the orphan nuclear receptor (Nr4a1) [20] transcription factor, that is also upregulated following dehydration in the SON [15].
Although Nr4a1 is upregulated in the SON following dehydration, our data shows that this mRNA is downregulated in the NIL. Further comparison of the SON and the NIL data reveals that there are 26 genes that are significantly regulated by greater than 1.5-fold in opposite directions as a result of dehydration (10 up in SON/down in NIL and 16 down in SON/up in NIL). It has been hypothesised that some transcripts that increase in abundance in the NIL as a consequence of
Hindmarch · Murphy
dehydration are in fact transported from the magnocellular cell bodies, down the axons to nerve terminals in the posterior pituitary. One such transcript encodes Avp [21] also identified here as being up-regulated by 2.5-fold in the NIL. The 16 transcripts identified, represent candidates for further study of anterograde axonal transport between the SON and the NIL. These data also identified 10 transcripts that are downregulated in the NIL but upregulated in the SON one of which, c-fos, has been suggested to be stored in the axons of the HNS and transported in a retrograde fashion in response to osmotic stimulation [22]. Whether the other 9 transcripts are subject to retrograde transport or just differently regulated in each tissue is not clear, but it is interesting to note that all three members of the orphan nuclear receptor subfamily (Nr4a1–3) are downregulated in the NIL, and Nr4a1 and Nr4a3 are both upregulated in the SON. Also interesting is the downregulation of pro-hormone convertases type 1 and 2 (Pcks 1 and 2) in the NIL and a corresponding upregulation in the SON following dehydration. The role of these proprotein convertases is to mediate post-translational modification of regulatory neuropeptides including provasopressin proinsulin, proglucagon, prosomatostatin, proCrf and Vgf. The expression of both PC1 and PC2 transcript has been observed in the magnocellular neurons of both the SON and PVN where both PCs are found in both Avp and Oxt neurons, while PC2 has also been observed in the parvocellular region of the PVN [23]. Interestingly, the same study showed that only PC2 mRNA was localised in Crf neurons suggesting specificity in processing.
Physiologists are adept at breeding particular traits into rodents that make them candidates for particular biological investigations. For example, marked differences in the hypothalamo-pituitary-adrenal axis (HPA) in different strains of rat exist [24] and the HPA of the inbred Wistar-Kyoto (WKY) rat strain, is particularly responsive to stress [25]. Outbred Sprague-Dawley (SD) and outbred Wistar rats differ in posterior pituitary weight and vasopressin gene product content, and whilst reserpine administration, which depletes catecholamine vesicles and inhibits vasopressin release, has no effect on PP Avp content in SD rats, it elicited a fall in Wistar animals [26]. Strain-dependent differences in sodium appetite and intake, and behavioural responses to salt excess, have also been reported [27, 28].
The wealth of transcriptome information that we have collected from various strains has afforded us the opportunity to re-mine old data with new questions. Data collected from ‘control’ SD, ‘control’ Wistar and ‘control’ Wistar-Kyoto (WKY) rats, each involved in a different experimental paradigm has been mined specifically to answer questions about the strain specific nature of transcriptome expression; the results are surprising. When data were arranged so that those genes whose expression is greater than 2-fold in either the SD-SON or SD-NIL or the WKY-SON or
The HNS Transcriptome Is Highly Strain Dependent
WKY-NIL were revealed, the expression of a large number of genes seems to be strain specific. In the SD-SON, 1,099 genes are enriched compared to the WKY-SON. Also, there are 374 genes that are enriched in the WKY-SON compared to the SD [29]. The same pattern of enrichment is noticed in the NIL too where 558 genes are SD specific and 309 genes WKY specific. The PVN data benefit from a third strain of rat, the Wistar. Interestingly, there are fewer enriched genes between the Wistar and the WKY-PVN than between SD-PVN, presumably reflecting the closer genetic relationship between these two strains.
In the light of this highly strain-dependent transcriptome expression we are left with an interesting issue to resolve. Given the apparent strain-dependent transcriptome expression noticed in the HNS, how is the highly conserved phenotype of osmoregulation achieved? Transcriptome-wide data are not currently available to satisfactorily answer this question; however, because dehydration results in an increase in vasopressin transcription, synthesis and discharge from the HNS in all strains of rats, we can hypothesise that the transcriptome expression between strains is more similar following dehydration than it is under ‘control’ conditions. The extension of this hypothesis is that the environmental conditions act as a phenotypic switch that aligns gene expression to a common purpose. The fitness of different strains to respond to such a switch will therefore be governed by the extent to which the straincommon or strain-unique genes are expressed. By analogy, the mixing of different primary gene colours will result in a strain-dependent spectrum of phenotypes that act as a substrate for selection pressure by the switch.
Conclusion
The analysis of transcriptome-wide data commonly results in lists of genes expressed in a particular tissue under different conditions with an emphasis placed on how the expression of each gene is changed beyond a prescribed cut-off. Interpretation of such data with a one-gene-at-a-time approach not only undermines the sentiment behind the approach but also ignores an important facet of the biological complexity that the researcher hopes to describe using that data. A disparity exists between the number of genes expressed by the human or rat genomes and the number of biological tasks they are required to achieve, a problem only resolved when one considers that the expression of any individual gene is part of a larger transcriptome-wide network of gene expressions rather than a discrete event. Work in our laboratory now seeks to describe the HNS transcriptome using this hypothesis, examining the relationships between all of the individual gene expressions to identify the role that the individual units of transcription play in the larger physiological response to osmotic stress.
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Charles Colin Thomas Hindmarch
Henry Wellcome Laboratories for Integrative Neuroscience and Endocrinology
Loche S, Cappa M, Ghizzoni L, Maghnie M, Savage MO (eds): Pediatric Neuroendocrinology. Endocr Dev. Basel, Karger, 2010, vol 17, pp 11–21
Role of Sleep and Sleep Loss in Hormonal Release and Metabolism
Rachel Leproult Eve Van Cauter
Department of Medicine, University of Chicago, Chicago, Ill., USA
Abstract
Compared to a few decades ago, adults, as well as children, sleep less. Sleeping as little as possible is often seen as an admirable behavior in contemporary society. However, sleep plays a major role in neuroendocrine function and glucose metabolism. Evidence that the curtailment of sleep duration may have adverse health effects has emerged in the past 10 years. Accumulating evidence from both epidemiologic studies and well-controlled laboratory studies indicates that chronic partial sleep loss may increase the risk of obesity and weight gain. The present chapter reviews epidemiologic studies in adults and children and laboratory studies in young adults indicating that sleep restriction results in metabolic and endocrine alterations, including decreased glucose tolerance, decreased insulin sensitivity, increased evening concentrations of cortisol, increased levels of ghrelin, decreased levels of leptin and increased hunger and appetite. Altogether, the evidence points to a possible role of decreased sleep duration in the current epidemic of obesity. Bedtime extension in short sleepers should be explored as a novel behavioral intervention that may prevent weight gain or facilitate weight loss. Avoiding sleep deprivation may help to prevent the development of obesity, particularly in children.
Hormones that Influence Glucose Regulation and Appetite Control Are Influenced by Sleep
The temporal organization of the release of the counterregulatory hormones growth hormone (GH) and cortisol as well as the release of hormones that play a major role in appetite regulation, such as leptin and ghrelin, is partly dependent on sleep timing, duration and quality. Glucose tolerance and insulin secretion are also markedly modulated by the sleep-wake cycle [1]. Sleep prop ensity and sleep architecture are in turn controlled by the interaction of two time-keeping mechanisms in the central nervous system, circadian rhythmicity (i.e. intrinsic effects of biological time, irrespective of the sleep or wake state) and sleep-wake homeostasis (i.e. a measure of the duration of prior wakefulness, irrespective of time of day).
Circadian rhythmicity is an endogenous oscillation with a near 24-hour period generated in the suprachiasmatic nuclei of the hypothalamus. The ability of the SCN nuclei to generate a circadian signal is not dependent on cell-to-cell interaction and synchronization. Instead, single SCN cells in culture can generate circadian neural signals [2]. The generation and maintenance of circadian oscillations in SCN neurons involve a series of clock genes (including at least per1, per 2, per3, cry1, cry2, tim, clock, B-mal1, CKIε/δ), often referred to as ‘canonical’, which interact in a complex feedback loop of transcription/translation [3, 4]. Circadian timing is transmitted to other areas of the brain and to the periphery via direct neuronal connections with other parts of the hypothalamus, via the control of sympathetic nervous activity and via hormonal signals, including melatonin. The molecular and neuronal mechanisms that measure the duration of prior wakefulness and are thus responsible for the homeostatic control of sleep have not been fully elucidated. Human sleep is comprised of rapid-eye-movement (REM) sleep and non-REM sleep. Deep non-REM sleep is characterized by ‘slow waves’ in the electroencephalogram (EEG), which reflect a mode of synchronous firing of thalamo-cortical neurons. The intensity of non-REM sleep may be quantified by slow wave activity (SWA; EEG spectral power in the 0.5–4 Hz frequency range). Slow waves of larger amplitude and greater regularity are reflected in higher SWA and in deeper sleep. Because SWA decreases in the course of the sleep period, is higher after sleep deprivation (i.e. extended wakefulness) and lower when the waking period has been interrupted by a long nap (i.e. shorter wakefulness), SWA is considered as the major marker of homeostatic sleep pressure. Converging evidence implicates adenosine, an inhibitory neurotransmitter, in sleep homeostasis in mammals [5]. Prolonged wakefulness results in increased levels of extracellular adenosine, which partly derive from ATP degradation, and adenosine levels decrease during sleep [6]. The adenosine receptor antagonist, caffeine, inhibits SWA [7]. It has been proposed that the restoration of brain energy during SWS involves the replenishment of glycogen stores [8]. The results of experiments testing this hypothesis have been mixed. A recent and well-supported hypothesis regarding sleep homeostasis is that the level of SWA in early sleep is a function of the strength of cortical synapses developed during wakefulness and that the decline in SWA across the sleep period reflects the downscaling of these synapses [9].
The major mechanisms by which the modulatory effects of circadian rhythmicity and sleep-wake homeostasis are exerted on peripheral physiological systems include the modulation of hypothalamic activating and inhibiting factors controlling the release of pituitary hormones and the modulation of sympathetic and parasympathetic nervous activity.
The relative contributions of the circadian signal versus homeostatic sleep pressure vary from endocrine axis to endocrine axis. It has been well-documented that GH is a hormone essentially controlled by sleep-wake homeostasis. Indeed, in men, the most reproducible pulse of GH occurs shortly after sleep onset, during slow wave sleep (SWS, stages 3 and 4) when SWA is high. In both young and older men, there
is a ‘dose-response’ relationship between SWS and nocturnal GH release. When the sleep period is displaced, the major GH pulse is also shifted and nocturnal GH release during sleep deprivation is minimal or frankly absent. This impact of sleep pressure on GH is particularly clear in men but can also be detected in women.
The 24-hour profile of cortisol is characterized by an early morning maximum, declining levels throughout the daytime, a period of minimal levels in the evening and first part of the night, also called the quiescent period, and an abrupt circadian rise during the later part of the night. Manipulations of the sleep-wake cycle only minimally affect the wave shape of the cortisol profile. Sleep onset is associated with a short-term inhibition of cortisol secretion that may not be detectable when sleep is initiated in the morning, i.e. at the peak of corticotropic activity. Awakenings (final as well as during the sleep period) consistently induce a pulse in cortisol secretion. The cortisol rhythm is therefore primarily controlled by circadian rhythmicity. Modest effects of sleep deprivation are clearly present as will be shown below.
The 24-hour profiles of two hormones that play a major role in appetite regulation, leptin, a satiety hormone secreted by the adipocytes, and ghrelin, a hunger hormone released primarily from stomach cells, are also influenced by sleep. The human leptin profile is mainly dependent on meal intake and therefore shows a morning minimum and increasing levels throughout the daytime culminating in a nocturnal maximum. Under continuous enteral nutrition, a condition of constant caloric intake, a sleeprelated elevation of leptin is observed, irrespective of the timing of sleep. Ghrelin levels decrease rapidly after meal ingestion and then increase in anticipation of the following meal. Both leptin and ghrelin concentrations are higher during nocturnal sleep than during wakefulness. Despite the absence of food intake, ghrelin levels decrease during the second part of the night suggesting an inhibitory effect of sleep per se. At the same time, leptin is elevated, maybe to inhibit hunger during the overnight fast.
The brain is almost entirely dependent on glucose for energy and is the major site of glucose disposal. Thus, it is not surprising that major changes in brain activity, such as those associated with sleep-wake and wake-sleep transitions, impact glucose tolerance. Cerebral glucose utilization represents 50% of total body glucose disposal during fasting conditions and 20–30% postprandially. During sleep, despite prolonged fasting, glucose levels remain stable or fall only minimally, contrasting with a clear decrease during fasting in the waking state. Thus, mechanisms operative during sleep must intervene to prevent glucose levels from falling during the overnight fast. Experimental protocols involving intravenous glucose infusion at a constant rate or continuous enteral nutrition during sleep have shown that glucose tolerance deteriorates as the evening progresses, reaches a minimum around mid sleep and then improves to return to morning levels [10, 11]. During the first part of the night, decreased glucose tolerance is due to decreased glucose utilization both by peripheral tissues (resulting from muscle relaxation and rapid hyperglycemic effects of sleep-
1971–1974
1976–1980
1988–1994
1999–2000
2001–2002
2003–2004
Fig. 1. Prevalence of overweight (>95th percentile) among American children and adolescents ages 2 to 19 years old from 1971 to 2004.
onset GH secretion) and by the brain, as demonstrated by PET imaging studies that showed a 30–40% reduction in glucose uptake during SWS relative to waking or REM sleep. During the second part of the night, these effects subside as light non-REM sleep and REM sleep are dominant, awakenings are more likely to occur, GH is no longer secreted and insulin sensitivity increases.
These important modulatory effects of sleep on hormonal levels and glucose regulation suggest that sleep loss may have adverse effects on endocrine function and metabolism. It is only during the past decade that a substantial body of evidence has emerged to support this hypothesis. Indeed, earlier work had only involved conditions of total sleep deprivation which are necessarily short term and therefore of dubious long-term clinical implication. The more recent focus on the highly prevalent condition of chronic partial sleep deprivation resulted in a major re-evaluation of the importance of sleep for health, and particularly for the risks of obesity and diabetes. In the two sections below, we first summarize the evidence from epidemiologic studies and then the evidence from laboratory studies.
Obesity and Sleep Loss: Epidemiologic Evidence
The increasing prevalence of obesity in both children and adults is affecting all industrialized countries. Figure 1 shows the change in the prevalence of overweight among American children per age category (2–5, 6–11 and 12–19 years) from 1971 to 2004 [12]. The prevalence of overweight went from about 5% in 1971 to about 15% in 2004 in each age category.
Increases in food intake and decreases in physical activity are the two most obvious reasons for the alarming increase in prevalence of obesity but experts agree that other factors must also be involved. Among those, reductions in sleep duration has
been proposed to be one of the most likely contributing factors [13]. Over the past few decades, nightly sleep duration (by self-report) has decreased in a mirror image with the increase in the prevalence of obesity. In 2008, the poll conducted by the National Sleep Foundation [14] revealed that American adults sleep on average 6 h 40 min during weekdays and 7 h 25 min during the weekend. In contrast, in 1960, the average sleep duration was 8.5 h [15]. Thus, over less than 50 years, a reduction of sleep duration by 1.5–2 h seems to have occurred. Short sleep durations seems to be also typical in American adolescents. Well-documented laboratory studies have shown that, when given a 10-hour opportunity to sleep for several days, children between 10 and 17 years of age sleep for about 9 h, indicating that sleep need is not less than 9 h [16]. In stark contrast with this physiologic sleep need are the sleep durations self-reported by American children between 11 and 18 years old in 2006 [17]. Even in the youngest children, the amount of sleep is less than 9 hours and drops to 7 h or less in 16- to 18-year-olds (fig. 2).
Is there an association between the prevalence of obesity and the prevalence of short sleep duration? Cross-sectional studies have examined associations between sleep duration and BMI in both children and adults and prospective studies have tested the hypothesis that short sleep duration at baseline predicted weight gain or the incidence of obesity over the follow-up period. All studies controlled for a variety of potential confounders. In adults, as of May 2009, a total of 29 cross-sectional studies and 6 prospective studies originating from a wide variety of industrialized countries have been published. Thirty of these 35 studies had positive findings. Obesity risk generally increased for sleep durations under 6 h. There have been 20 cross-sectional
Fig. 2. Self-reported sleep duration in American adolescents in 2004.
Table 1. Prospective studies of sleep (reported by the parents) and obesity risk in boys and girls.
Reference
Number of subjects and years of follow-up
Lumeng et al. [18], 2007 n = 785
aged 9–10 years (3rd grade) and 11–12 years (6th grade)
Agras et al. [19], 2004 n = 150
sleep reported at 3-5 years
weight measured at 9.5 years
Reilly et al. [20], 2005 n = 7,758
sleep reported at 38 months
obesity measured at 7 years
Taveras et al. [21],
2008 n = 915
sleep reported at 6 months, 1 year and 2 years
BMI z score measured at 3 years
Touchette et al. [22],
2008 n = 1,138
sleep duration reported yearly from 2.5 to 6 years
BMI measured at 2.5 and 6 years
Sugimori et al. [23], 2004 n = 8,170 sleep and BMI measured at ages 3 and 6 years
Snell et al. [24], 2007 n = 2,281
aged 3–12 years at baseline and 5 years later
Results
short sleep duration in 3rd grade is associated with overweight in 6th grade
less sleep time in childhood is a risk factor for childhood overweight
short sleep duration (<10.5 h) at age 3 years is associated with a risk of obesity
short sleep duration (<12 h/ day) during infancy is associated with a higher BMI z score at 3 years
persistent short sleepers (<10 h) increases risk of overweight and obesity in later childhood
short sleep duration (≤9 h) is associated with a risk of obesity in boys, not in girls
less sleep is associated with higher BMI, 5 years later
Country of origin
USA
USA
UK
USA
Canada
Japan
USA
studies in children and all had positive findings. Prospective studies are particularly important because they provide an indication regarding the direction of causality. Also, an overweight child is at higher risk of becoming an overweight or obese adult. Table 1 summarizes the 7 prospective epidemiologic studies so far that have examined sleep duration and obesity risk in boys and girls. All 7 studies showed a significant association between short sleep duration at baseline and weight gain or incidence of overweight or obesity over the follow-up period. In conclusion, the epidemiologic data consistently support a link between short sleep and obesity risk. Negative studies were mostly focusing on older adult populations. Of note, two cross-sectional studies used objectively recorded sleep, rather than self-report, and also found a significant association between short sleep and higher BMI. A major limitation of nearly all these studies is that there was no assessment of sleep quality or sleep disorders and therefore it is generally not known if short sleep
was the result of bedtime restriction in a healthy sleeper or of the inability to achieve more sleep in an individual suffering from a sleep disorder.
Epidemiologic studies in adults have also shown associations between short sleep and diabetes risk [25]. Studies are needed to determine if the increased prevalence of type 2 diabetes in children and young adults is also partly predicted by short sleep.
Obesity, Diabetes and Sleep Loss: Evidence from Laboratory Studies
There has been no laboratory study so far that has examined the impact of experimental recurrent sleep restriction on hormones and metabolism in children. The existing laboratory studies were all conducted in young to middle-aged adults.
The first well-controlled laboratory study that tested the hypothesis that partial sleep deprivation could affect the metabolic and endocrine function was published 10 years ago [26]. Young lean subjects were studied (1) after building a state of sleep debt by restricting bedtime to 4 h for 6 nights, (2) after full recovery, obtained by extending the bedtime period to 12 h for 7 nights, and (3) under normal condition of 8 h in bed. This latter 8-hour bedtime condition was performed 1 year after the two other sleep conditions. Figure 3 shows the 24-hour profiles of leptin, cortisol, GH and HOMA (homeostatic model assessment, an integrated measure of glucose and insulin, that is the product of glucose concentration (mmol/l) by insulin concentration (mIU/l) divided by 22.5) under the 3 bedtime conditions. Caloric intake was the same in the 3 conditions, i.e. 3 identical carbohydrate-rich meals. Posture and physical activity were also controlled as continuous bed rest was enforced during blood sampling. Clearly, overall leptin levels, evening cortisol levels, and the HOMA response to breakfast varied in a dose-response relationship with sleep duration. Shorter sleep duration was associated with greater disturbances in these hormonal and metabolic variables. Leptin levels were lowest when the subjects were in a state of sleep debt, signaling the brain an unnecessary need for extra caloric intake. Evening cortisol levels were highest when the subjects were in a state of sleep debt. A state of sleep debt therefore appears to delay the normal return to low levels of corticotropic activity. HOMA levels post-breakfast were the highest in a state of sleep debt indicating a decrease in glucose tolerance and/or a decrease in insulin sensitivity. The 24-hour GH profiles in the 8- and 12-hour bedtime conditions were qualitatively similar, with a trend for lower post-sleep peak values in the extended bedtime that is consistent with a reduced homeostatic drive for sleep with the decreased duration of the wake period. In the state of sleep debt, a GH pulse prior to sleep onset was observed, in addition to the normal post-sleep onset GH pulse. The elevation of GH concentrations during waking could have an adverse impact on glucose metabolism.
A subsequent study [27] examined appetite regulation after 2 nights of 4 h in bed and after 2 nights of 10 h in bed, in a randomized cross-over design. This study confirmed the decrease in leptin levels seen in the previous study, with a 18% decrease of
leptin levels after the short nights relative to the long nights. Furthermore, ghrelin was assayed and showed a 28% increase after the 2 nights of 4 h in bed. Questionnaires on hunger and appetite were completed and indicated a 24% increase in hunger and a 23% increase in global appetite after the 4-hour nights versus the 10-hour nights. Appetite for high carbohydrate nutrients was the most affected with a 32% increase. Importantly, the subjective report of increased hunger was correlated with the increase in ghrelin to leptin ratio (i.e. hunger factor/satiety factor). These observations suggest that in real life, when food is available everywhere and all the time, sleep deprived people may consume excessive amounts of calories, particularly from carbohydrates. A recent study tested this hypothesis using a randomized cross-over design with either extension or restriction of the usual bedtime period by 1.5 h for 2 weeks in the laboratory [28]. The subjects were middle-aged overweighed individuals who were exposed to unlimited amounts of palatable food presented in 3 meals per day and snacks were continuously available. The volunteers consumed excessive amounts of calories from meals under both sleep conditions but consumed more calories from snacks when sleep was restricted rather than extended.
Cortiso
Fig. 3. Relationship between sleep duration and leptin, cortisol, GH and HOMA.
Table 2. Alterations in glucose metabolism after sleep loss: 2 laboratory studies
Several studies have also shown that recurrent partial sleep restriction or experimentally reduced sleep quality results in decreased insulin resistance, another risk factor for weight gain and obesity. Remarkably, the decrease in insulin sensitivity was not associated with a compensatory increase in insulin release, and therefore diabetes risk was elevated.
The upper part of table 2 presents a re-analysis of the data from intravenous glucose tolerance testing (ivGTT) performed in the initial ‘sleep debt study’ [26] after 5 days of bedtime restriction to 4 h per night and when the subjects were fully rested at the end of the recovery period. Glucose tolerance was decreased by more than 40% when the subjects were in the state of sleep debt. This may be partly due to a decrease in brain glucose utilization as Sg (glucose effectiveness) which quantifies non-insulin-dependent glucose disposal, was significantly reduced. Insulindependent glucose disposal was also decreased since the glucose disposition index was markedly lower. Consistent findings have been observed in several follow-up studies [29, 30].
Recently, a study showed that reduced sleep quality, without change in sleep duration, can also have adverse effects on glucose metabolism [31]. Slow-wave sleep was suppressed by delivering acoustic stimuli that replaced deep sleep SWS by shallow NREM sleep (stage 2) for 3 consecutive nights, mimicking the impact of four to five decades of aging. The lower part of table 2 shows the results of an ivGTT performed at baseline and after 3 nights of SWS suppression. The findings are qualitatively similar to those seen after 5 nights of bedtime curtailment but of lesser magnitude as would be expected since the intervention was of shorter duration.
Conclusions
Rapidly accumulating evidence suggests that sleep disturbances, including insufficient sleep due to bedtime curtailment and poor sleep quality, may represent novel risk factors for obesity and type 2 diabetes. While laboratory studies have been conducted in adults only, a large number of epidemiologic studies in pediatric populations have demonstrated associations between short sleep and adiposity that are often stronger than those seen in adult populations. Sleep curtailment appears to be an increasingly prevalent behavior in children and, in the United States, adolescents may well be the most sleep-deprived age group with a difference between self-reported sleep and estimated sleep need of more than 2 h daily. There is a paucity of knowledge regarding how insufficient sleep and sleep disorders may affect pubertal development and growth, despite the fact that it has been known for several decades that the release of sex steroids and GH is markedly dependent on sleep during the pubertal transition. An increasing number of children are obese and may suffer from obstructive sleep apnea. The impact of this sleep disorder, which is known to promote insulin resistance and reduced testosterone in adults, on neuroendocrine release and metabolic function in children is in urgent need of rigorous study.
Acknowledgements
Part of the work described in this article was supported by US National Institute of Health grants P01 AG-11412, R01 HL-075079, P60 DK-20595, R01 DK-0716960, R01 HL-075025 and M01 RR000055 and by US Department of Defense award W81XWH-07–2-0071.
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Sexual Hormones and the Brain: An Essential Alliance for Sexual Identity and Sexual Orientation
Alicia Garcia-Falgueras Dick F. Swaab
Netherlands Institute for Neuroscience, an Institute of the Royal Netherlands Academy of Arts and Sciences, Amsterdam, The Netherlands
Abstract
The fetal brain develops during the intrauterine period in the male direction through a direct action of testosterone on the developing nerve cells, or in the female direction through the absence of this hormone surge. In this way, our gender identity (the conviction of belonging to the male or female gender) and sexual orientation are programmed or organized into our brain structures when we are still in the womb. However, since sexual differentiation of the genitals takes place in the first two months of pregnancy and sexual differentiation of the brain starts in the second half of pregnancy, these two processes can be influenced independently, which may result in extreme cases in transsexuality. This also means that in the event of ambiguous sex at birth, the degree of masculinization of the genitals may not reflect the degree of masculinization of the brain. There is no indication that social environment after birth has an effect on gender identity or sexual orientation.
Sex Differences in Cognition and Aggression: Little Effect of the Social Environment
Boys and girls behave in different ways and one of the stereotypical behavioral differences between them, that has often been said to be forced upon them by upbringing and social environment, is their behavior in play. Boys prefer to play with cars and balls, whereas girls prefer dolls. This sex difference in toy preference is present very early in life (3–8 months of age) [1]. The idea that it is not society that forces these choices upon children but a sex difference in the early development of their brains and behavior is also supported by monkey behavioral studies. Alexander and Hines [2], who offered dolls, toy cars and balls to green Vervet monkeys found the female monkeys consistently chose the dolls and examined these ano-genitally, whereas the male monkeys were more interested in playing with the toy cars and with the ball. ‘Neutral’
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